Chloroplast-mitochondria synergy modulates responses to iron limitation in two Thalassiosira diatom species

Iron is naturally present at low levels in modern oceans, where it is essential for life. Ocean-dwelling organisms such as oceanic phytoplankton must therefore adapt to the available levels. Among phytoplankton, diatoms are a highly diverse and successful taxon that includes the Thalassiosira genus. As a group, diatoms contribute around 20% of global primary photosynthetic productivity; they have also developed specific resources allowing them to thrive in low-iron regions. However, the major biological factors underlying their success in these ocean environments remain unknown. Here, we compared two Thalassiosira species: Thalassiosira oceanica from iron-poor open-ocean; and Thalassiosira pseudonana, from iron-rich coastal waters. Since iron is essential for both photosynthesis and respiration, we examined the specificities of the bioenergetic machineries in these organisms using a combination of photo-physiological, proteomics, and focused ion beam scanning electron microscopy methods. We particularly focused on chloroplast-mitochondrial coupling, a mechanism deployed by diatoms to ensure optimal transfer of photosynthetic products to promote cell growth. This study of this mechanism in the context of iron limitation reveals that the two diatoms differentially remodel chloroplast compartments in response to iron limitation. Their tolerance to these conditions is also linked to distinct constitutive mitochondrial architectures.